The invention relates to a brake pad for a disc brake, in particular for a commercial vehicle disc brake which can be installed in 19.5″ and 22.5″ wheels.
Disc brakes fitted with such brake pads are used mainly in commercial vehicles, wherein the brake wear during long-distance operation occurs primarily by way of frequent adaptive braking which, admittedly, is carried out at relatively high traveling speeds but with low to average brake pressures. Rarely occurring emergency braking with high braking forces has only a very insignificant influence upon the overall wear of the brake.
The force initiation upon the brake pad is carried out via the pressure pistons (tappets) which, according to the conventional designs, bear against pressure pieces which are molded on the lining support plate.
In this case, the position of the pressure pieces or of the pressure pistons is predetermined by the construction-related circumstances and the installation conditions.
As a result of a non-optimum positioning of the force initiation zones, an unfavorable pressure distribution arises in the lining friction surface and cannot be adequately compensated either by additional pressure plates or by particularly thick-walled pad support plates. A particularly disadvantage consequence of this, on the one hand, is a relatively high weight, which contrasts with a continuous demand for weight optimization and, on the other hand, is an uneven wear of the brake pad and brake disc. Naturally, the service life of the disc brake is shortened as a result, which leads to increased operating costs.
The invention is therefore based on the object of further developing a brake pad so that its service life is improved.
This and other objects are achieved by a brake pad for a disc brake which can be installed in a wheel with a size of 19.5″, with a ring segment-like pad support plate carrying a friction lining material the inner radius of which merges on both sides into straight bearing surfaces and which on its rear side facing away from the friction lining material has two molded-on pressure pieces which are arranged at a distance from each other. The length of the pad support plate is between 205 and 220 mm, the inner radius is between 100 and 125 mm, and the outer radius is between 185 and 200 mm.
As a result of the dimensional specifications both of the positioning of the pressure pieces and of the lengths and radii dimensions of the pad support plate, equal wear conditions are created over the entire friction lining surface, and also a weight minimization of the pad support plate is achieved with the best possible pressure distribution at the same time.
For determining the specifications according to the invention, the two brake lining halves, which are formed mirror-symmetrically transversely to the circumferential extent of the ring segment-like pad support plate, are considered separately.
In this case, the area centroid, which in a further consideration represents the reference point for the fixing of the pressure piece, is determined in each case. A force initiation directly in the area centroid, with ideal rigidity of the pad support plate, would create a homogeneous pressure distribution in the friction surface.
However, the lining wear is predetermined in a first approximation by the converted braking energy. If the different regions of the friction surface are considered from this point of view, then it shows that the radially outer surface regions, at the higher running speed which exists there, with the same surface pressure in the lining friction surface, have a higher energy density than the radially inner regions of the friction surface.
This knowledge leads to the fact that for achieving a uniform wear behavior, the surface pressure should not be constant over the entire lining friction surface but the energy density, i.e. the specific braking power (Nspez), formed from the product of surface pressure (p)·friction coefficient (μ)·running speed (v), should be constant, wherein the aim is for the braking power (Nspez) to be constant.
This relationship is influenced by the fact that the lining friction coefficient likewise experiences changes influenced by pressure, running speed and temperature. Since the overall wear is decisively determined by adaptive braking, as was described above, during which the changes induced by pressure and temperature are still small, the additional consideration of these influencing variables can be dispensed with.
The influence of different running speeds, however, is distinctly present. In the case of the relatively small braking forces of a typical adaptive braking with about 1.5 bar brake pressure, pressures over the friction surface of between 150 and 200 N/cm2 are achieved. Under these conditions and with a ratio of the running speed on the outer friction ring diameter to that on the inner friction ring diameter of 1.6, the ratio of the friction coefficient on the outer friction ring diameter to that on the inner friction ring diameter is about 0.8, i.e. with homogenous pressure distribution, the generated friction power on the outer diameter of the friction strip is 1.6×0.8=1.28 higher than on the inner friction ring diameter. The factor of 1.28 is therefore to be compensated by varied pressure distribution.
This occurs by the force initiation position of the pressure pistons, therefore of the pressure pieces, in relation to the position determined for uniform pressure distribution on the connecting line of the area centroids of the lining halves, being displaced radially towards the axial center by an amount which is to be correspondingly fixed. The value of this amount is dependent upon the dimensions of the brake disc and brake pad and also upon the shape of the brake pad. For a brake for 22.5″ rim size, the dimension is between 1 mm and 6 mm. In field trials, values of about 3 mm have proved to be especially suitable.
Another factor which influences the pressure distribution and therefore the energy density over the friction surface is the elasticity of the pad support plate, which is to distribute the force initiated by the pressure pistons into a uniform surface pressure.
Since the pad support plate yields under the surface pressure of the lining in the regions which are at a distance from the force initiation zones, a curvature is created and assumes maximum values in the lining center and on the tangentially oriented ends of the brake pad. Beneath the more sharply curved zones of the pad support plate, a lower surface pressure inevitably prevails since the friction lining mass which is compressed under the pressure partially expands.
This curvature has been minimized up to now by means of correspondingly stably dimensioned and therefore also correspondingly largely dimensioned pad support plates or additional pressure plates.
One possibility for achieving a smallest possible deflection of the pad support plate yet with low weight exists in an optimum positioning of the pressure pieces in the tangential direction. A minimum deflection is specifically achieved if the force initiation position is selected on a connecting line according to the condition Nspez, (braking power)=surface pressure (p)·friction coefficient (μ)·running speed (v)=constantly fixed force initiation positions, so that the deflection in the lining center and at the tangential ends of the lining is of equal value. Trials carried out on a disc brake for 22.5″ wheels showed that an arrangement of the force initiation positions at the area centroid of the lining halves displaced tangentially outward by between 3 and 15 mm gives the best possible pressure distribution.
The pressure pieces, which are molded onto the pad support plate, are of a disc-like or ring-like design and conduct heat from the brake pad into the components of the brake application device. Since, as a result of this, an undesirable loading of the corresponding components occurs, the contact surface between the respective pressure piston and the bearing surface of the pressure piece is selected to be just large enough for no plastic deformation to occur on the pressure pistons and/or on the bearing surfaces of the pressure pieces even with maximum temperature on the brake pad in the event of full braking. Depending upon the material of the pad support plate, bearing surfaces per pressure piece of between 15 and 35 cm2 have proven to be adequate. In the case of cast pad support plates, the bearing surfaces can lie in the lower range of the specified values on account of the high resistance to pressure of the cast material.
In the case of disc brakes for 22.5″ wheels, the bearing surface has a diameter of 55 mm, whereas the overall thickness in the bearing region is 14 mm, with a thickness of the pad support plate of 9 mm in the adjacent surrounding region.
For establishing the operating parameters, the conditions of an adaptive braking are taken as a basis, namely a brake pressure of between 1.0 and 2.0 bar, a speed of between 60 and 90 km/h and a brake temperature of between 100 and 200° C.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.